In many applications, particularly in medical and aerospace fields, minimizing volume and weight is a major goal in battery design. As battery technology continues to make great strides, battery sizes have greatly decreased. Because of size and weight constraints, the number of available materials used for various battery components is decreasing. Furthermore, when providing batteries for a replacement market, the size of the battery is constrained by the available space in the existing device. For example, for replacement batteries for certain models of a hearing aid already in use, the battery thickness is limited to 3.6 mm, and the diameter is 5.8 mm. In order to not reduce the capacity of the cell, the space taken up by nonreactive components, such as the battery case and sealing components, must be minimized, thus reducing the amount of room available to seal the battery case.
Lithium-ion batteries provide high energy densities; however, a major problem associated with these cells is the highly corrosive nature of lithium battery chemistry. Hermetic seals are used to protect living tissue from corrosive battery components and to protect battery components from corrosive bodily fluids. Hermetic seals must be manufactured as ruggedly as possible for applications where hermeticity will be required for extended exposures to harsh environments.
Electronic device seals that bond glass to metal are generally known in the art. Molecular bonding is accomplished by oxidizing the surface of the metal component to facilitate bonding to the glass component. Heating the components causes the glass to soften and flow into the oxidized area of the metal component thereby creating a hermetic seal when the components are cooled. For typical feedthrough constructions using a glass as the insulator, a compression seal is created, for example, where an outer body (typically a metal case) has a coefficient of thermal expansion (CTE) that is greater than that of an insulating component (typically glass), and the insulating component has a CTE that is greater than that of a metal component (typically a pin). Once heated to 950° C. or greater, the differing CTE facilitates the glass flowing into the case to form a seal, and likewise, the glass to compress the pin to form yet another seal. It is desirable for the glass and metal to have similar CTE to avoid stress breaks during the heating and cooling processes. Thermal expansion is particularly problematic where the CTE of the battery case material differs substantially from that of the pin or insulator material.
Therefore, to form an acceptable glass-to-metal seal in a lithium or lithium-ion battery, the glass must have a high resistance to lithium corrosion; it must be able to make a hermetic seal between the metal header and the metal pin, which requires a thermal expansion match between the glass and the pin; and it must be an electrical insulator so that the case cover and the pin are electrically isolated. Also, where feedthroughs may come into contact with bodily fluids, it is necessary to choose biostable materials.
To manufacture a battery, typically, an electrode assembly is placed in a case having a cover. To keep weight at a minimum, it is desirable to use strong, yet lightweight materials for the battery case and cover. These materials may, as an example, include titanium and titanium alloys. However, titanium presents problems in most applications in that its CTE varies greatly from materials traditionally used for the feedthrough pin, resulting in seal failures.
The battery case is hermetically sealed to prevent corrosion and to avoid leakage of the internal electrolyte, which is typically very corrosive. Because of corrosion issues, only a limited number of materials can be used in contact with the electrolyte. For the positive feedthrough of a lithium ion battery, these materials include aluminum, platinum, gold, niobium, tantalum, molybdenum, and stainless steel. Because the CTE of the desirable battery cover material, e.g. titanium, is generally markedly different from the CTE of desirable pin material, e.g., stainless steels that can withstand electrolyte exposure, these materials tend to expand and contract at differing rates. The CTE of the insulating member may be different from that of one or both components as well. These differences in CTE make it difficult to form a good seal between the insulating body and the case or terminal pin during manufacturing, or may cause the seal to break during use.
To prevent these problems, the prior art has generally called for the requirement of materials that have compatible CTEs. As mentioned previously, a compression seal can be formed when the CTE for the pin material is less than that of the battery cover material. A quick look at stainless steel CTE reveals that these CTEs are larger than that for titanium and Ti-6A1-4V alloy, essentially eliminating this combination of materials for forming a glass compression seal.
TABLE 1shows the CTE of various materials.CTE[10−6/° C.]ConductorsAluminum23.51000 series (1004)Gold14Au 100Nickel42 Alloy4.7Kovar (Co17, Ni29)6PlatinumPt 1009PtIr9.2Stainless Steel30417.2304L17.230517.231615.9316L15.94109.942010.344610.4TitaniumTitanium CP8.4Ti 6AL-4V8.8InsulatorsNonglass Ceramics7.6Al2O3Glass6.7CaBAl 12
Furthermore, the compression seal described above requires a minimum thickness for the various components. For applications in which the overall thickness of the battery is limited, such as in the hearing aid replacement battery market, there is simply not enough room allotted to the feedthrough to provide the thickness of material necessary to form a strong glass compression seal.